I. Gross Anatomy and Histology

Objectives:

After completing this tutorial the student will be able to:

list and identify the various parts of the kidney

describe the gross anatomy and histology of the kidney

Anatomy

The kidneys are two somewhat bean-shaped, reddish-brown organs located
in the retroperitoneal space on either side of the vertebral column.
Each kidney weighs about 150 gms, measures 11 cm long and 6 cm wide
and lies between T11-12 and L3. Each kidney has a convex lateral border
and a concave medial border with the two borders merging at the poles
(superior and inferior portions). Surrounding each organ is the fibrous
renal capsule, which is loosely adherent to it. Adipose tissue
surrounds the capsule and is in turn surrounded by the renal fascia
(Gerota's fascia), which secures the kidney to the posterior
abdominal wall. Much of the medial border is occupied by an indentation,
the hilum, through which the renal vessels, nerves, lymphatics
and the renal pelvis enter or leave the renal sinus, the space
enclosed by the renal parenchyma.

The bisected kidney through the hilum shows the parenchyma to consist
of an outer cortex, which forms a continuous subcapsular band
of tissue, and an inner medulla, which is discontinuous being
interrupted by projections of the cortex towards the renal sinus, the
renal columns (columns of Bertin). The medulla consists
of several triangular structures, the pyramids, with their bases
towards the cortex and their tips, called papillae, projecting
into minor calyces.

Question: What is the name of the functional and structural
unit of the kidney?

Question: Into what regional nodes do the renal lymphatics drain?

Histology

The renal parenchyma is composed of functional units called the nephron,
and connective tissue, the interstitium.

Each nephron consists of a tuft of anastomosing capillaries called
the glomerulus, formed from the afferent arteriole and draining
into the efferent arteriole, and a tubular system called the renal
tubule. Epithelial cells called podocytes (or visceral
epithelium of Bowman's capsule) invest the glomerulus, and are reflected
to become continuous with the parietal epithelium of the Bowman's
capsule. The Bowman's capsule is the bulbous, distended, closed
proximal end of the tubular system and is invaginated by the glomerulus.
The space between the glomerulus and capsule is the urinary space.

Extending from the capsule is the proximal tubule, which is
lined by tall cuboidal-to-columnar epithelial cells containing many
mitochondria and a prominent brush border. The proximal tubule is the
longest portion of the tubular system and is made up of convoluted
proximal and distal straight (pars recta) segments. The pars
recta descends into the medulla, where it forms the U-shaped loop of
Henle. The latter reenters the cortex within which it forms the straight
and convoluted segments of the distal tubule. The distal tubule,
at about the junction between its two segments, runs close to the glomerular
hilum and forms a specialized segment called the macula densa
(see below). The distal tubule is lined by cuboidal epithelium that
lacks a brush border.

Low magnification view of a section through the kidney. The cortex is
the more darkly staining region at the top (outside of the kidney).
the lower lightly stained tissue is the renal medulla ending in the
pointed renal papilla. urine empties into the Y-shaped space made up
of the renal calyces (the arms of the Y) and the pelvis (the base of
the Y).

Section through the cortex showing a group of cross-sectioned profiles
of proximal (p) and distal (d) tubules. Both are composed of simple
cuboidal epithelium but the epithelium of the proximal tubule is taller,
has prominent brush border and contains clear vacuoles.

Question: Which hormone acts on the distal convoluted tubules?

Question: What are the parts of the loop of Henle?

The distal tubule empties into collecting tubules, which in
turn drain into collecting ducts. The latter converge, as they
approach the medulla, to form the collecting ducts of Bellini,
which run vertically through the medulla to the papillae. The collecting
tubules and ducts are lined by pale-staining, cuboidal epithelial cells
called clear cells scattered amongst which are darker-staining intercalated
cells.

Section through the renal papilla that extends into the renal calyx.
The papilla contains a number of large, parallel ducts called papillary
collecting ducts (ducts of Bellini).

Longitudinal view of papillary collecting duct (duct of Bellini). The
duct is lined by simple cuboidal epithelium with definite cell borders
(not seen in proximal or distal tubules).

The straight portions of the proximal and distal tubules and the collecting
ducts run in parallel arrays in a portion of the cortex devoid of glomeruli
and inaccurately called the medullary rays (because they appear
to emanate from the medulla). The medullary ray and the glomeruli and
convoluted tubules on either side of it form the cortical subunit, the
lobule.

The interstitium is made up of the interstitial cells, which are fibroblast-like,
and matrix. In the cortex the interstitium is small and mainly occupied
by small blood vessels and lymphatics and, at the hilum of glomeruli,
lacis cells. However, in the medulla it increases considerably in amount.
The lacis cells, together with the macula densa and specialized myoepithelial
cells in the walls of afferent and efferent arterioles containing neurosecretory
granules filled with renin form the juxtaglomerular apparatus.

Question: What is the role of the renal medullary interstitium
in urine production?

Department of Pathology

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